Actually, yes. The forum log you out if it "feels" you stay inactive too long. Even when you just seem inactive because you're writing a long post. I've had to log in again after writing a message before the system let me post it with a generous "Proceed".

The thing to do is to check "Remember me". I guess then the system wouldn't throw you out. At least I haven't had it happen to me yet.

And then at other times you can't log out at all... I've had that too.